ME450 Fall 06
Redesigning High-End Office Chair
FINAL REPORT
Team: MOSAIC
Hyung Min Chae
Xunlei Li
Christopher Liong
Jinjin Ma
Section instructor: Prof. Sridhar Kota
Date: 12/15/06
EXECUTIVE SUMMARY
The Knoll Life Chair Series 400 is a high-end office chair with a mechanism that couples the motion of the
seat with the recline angle of the chair’s back which is a well-liked feature among customers. However, the
current mechanism is relatively heavy, complicated, and difficult to assemble. Due to the high cost associated
with these characteristics, the Knoll Life Chair Series 400 office chair is not as price competitive and as
profitable as it can be in the market. Thus we were asked by the Knoll Furniture Company to modify the
current chair design such that the chair’s 1) current functions are maintained, 2) number of parts is reduced, 3)
the overall cost of the chair is reduced, 4) modified mechanisms can be easily integrated onto the current chair,
and 5) reliability is maintained.
Based on the customer’s requirement, we obtained a list of general engineering specifications for our design
which are 1) simplification of synchronized recline and upright tilt locking mechanism, 2) maintenance of all
current functions, 3) capable of entering full-scale production, 4) easy to assemble, 5) high reliability. Using
QFD analysis, we ranked the importance of the customer requirements and the general engineering
specifications.
Based on the engineering specifications identified, we generated a total of nine design concepts focused on
redesign of 1) the synchronized recline mechanism, 2) complicated upright tilt locking mechanism and 3) the
back casting to allow for ease of assembly. Based on results obtained from our Pugh chart study, for the
synchronized recline mechanism, we chose a concept which transforms the current four-bar linkage system of
the synchronized recline mechanism into a 4-link compliant mechanism with Small Linkage Flexural Pivot
(SLFP) replacing the original pin joints as the final concept for our alpha design. The torsion bar in the current
chair design that stored energy during recline movement of the chair was eliminated and replaced by the
compliant mechanism. Using this final concept we were able to develop an initial design that was optimized to
support a weight of 200 lb and allow for a back inclination angle of ±16 degrees. We determined that E-glass
SLFP segments and Aluminum rigid links would provide the highest safety factor for our design among the
materials that we analyzed. For the upright tilt lock system, we chose to redesign it to a one-piece compliant
mechanism using High Density Poly Ethylene (HDPE). Using this method, we would be able to reduce the
parts dramatically while sustaining the current function.
Based on the CAD drawings for both designs, the manufacturing plan was generated. According to the plan,
we built two versions of the compliant synchronized recline mechanism using water jetting. Prototype One is a
one-piece aluminum mechanism which has partial functionality. Prototype Two is a fully functional compliant
synchronized recline mechanism based on our E-glass and Aluminum dual-material design which has allows
for a full range of chair motion. We built the compliant upright tilt lock mechanism based on our CAD
drawing using High Density Polyethylene (HDPE) from the laser cutter.
Compared to the original product, we reduced the synchronized recline and the upright tilt-lock mechanisms’
part counts from 30 to 21 parts and 9 to 3 parts, respectively. The four-bar linkage mechanism on the current
chair has a vertical displacement of 1.55 cm and the current upright tilt-lock mechanism has horizontal
displacements of 1.2 cm at its ends. Prototype Two, the compliant synchronized recline prototype with EGlass SLFP segments, had a vertical displacement of 1.2 cm and the compliant upright tilt-lock prototype had
horizontal displacements of 0.7 cm on each side. Based on these results, we have achieved the goals of the
project because we reduced the number of parts which will in turn reduce the cost of the current chair. We
also maintained the current functions of the chair because the results we received from our prototype were
similar to the current chair’s attributes.
However, our design for the synchronized recline mechanism is not a one-piece compliant mechanism due to
the high cost of manufacturing the entire system using E-glass. Therefore, this may cause some potential
problems for full-scale manufacturing of the product. In the future, the development of a distributed compliant
one-piece mechanism should be explored. We also recommend the future teams to try and integrate the
designs onto the chair. To integrate the designs, 3-D force and stress analysis based on the current chair should
be developed
1
Table of Contents
EXECUTIVE SUMMARY .........................................................................................................................................1
TABLE OF CONTENTS ............................................................................................................................................2
INTRODUCTION .......................................................................................................................................................3
CORRESPONDING ENGINEERING SPECIFICATIONS ...................................................................................4
SPECIFIC DESIGN CHARACTERISTICS .....................................................................................................................5
FUNCTIONAL DECOMPOSITION.........................................................................................................................6
SYNCHRONIZED RECLINE MECHANISM .....................................................................................................................6
UPRIGHT TILT LOCK MECHANISM ..........................................................................................................................7
CONCEPT GENERATION & CONCEPT SELECTION.......................................................................................7
CONCEPTS FOR SYNCHRONIZED RECLINE MECHANISM ........................................................................................7
CONCEPTS FOR UPRIGHT TILT-LOCK MECHANISM...............................................................................................9
DESIGN FOR EASY ASSEMBLY ...............................................................................................................................10
FINAL CONCEPT SELECTION ............................................................................................................................10
ENGINEERING DESIGN PARAMETER ANALYSIS ........................................................................................11
FINAL DESIGN DESCRIPTION............................................................................................................................18
PROTOTYPE DESCRIPTION .......................................................................................................................................18
FINAL DESIGN ........................................................................................................................................................19
COMPLIANT SYNCHRONIZED RECLINE MECHANISM ...........................................................................................19
COMPLAINT UPRIGHT TILT LOCK MECHANISM ..................................................................................................31
MANUFACTURING PLAN.....................................................................................................................................33
COMPLIANT SYNCHRONIZED RECLINE MECHANISM ...........................................................................................33
COMPLIANT UPRIGHT TILT LOCK MECHANISM ..................................................................................................35
TEST RESULTS........................................................................................................................................................35
COMPLIANT SYNCHRONIZED RECLINE MECHANISM ...........................................................................................36
UPRIGHT TILT-LOCK MECHANISM .......................................................................................................................36
DESIGN CRITIQUE.................................................................................................................................................37
SYNCHRONIZED RECLINE COMPLIANT MECHANISM ...........................................................................................37
UPRIGHT TILT LOCK COMPLIANT MECHANISM ..................................................................................................38
INFORMATION SOURCES....................................................................................................................................38
BENCHMARKING ....................................................................................................................................................38
CONCLUSIONS........................................................................................................................................................39
RECOMMENDATIONS ..........................................................................................................................................39
ACKNOWLEDGEMENTS ......................................................................................................................................40
REFERENCES ..........................................................................................................................................................40
APPENDIX A. FIGURE A1 GANTT CHART DIAGRAM ........................................................................................41
APPENDIX B. PRELIMINARY DESIGN CONCEPTS ...................................................................................................42
APPENDIX C. DIMENSION ANALYSIS FOR SLFP SEGMENTS OF COMPLIANT SYNCHRONIZED RECLINE
MECHANISM ...........................................................................................................................................................47
APPENDIX D. FORCE ANALYSIS FOR EACH PIVOT JOINT USING ADAMS MODEL ...............................................49
APPENDIX E. BILL OF MATERIAL .........................................................................................................................50
2
INTRODUCTION
Knoll is internationally renowned for creating
innovative and modern office furnishings. Started in
1938, there are over 300 Knoll dealerships and 100
showrooms and regional offices in North America
alone. Knoll also has showrooms and dealers located
in Europe, Asia, and Latin America. Knoll’s best
selling product is the Knoll Life Chair Series 400 office
chair, as shown in Figure 1. Although the chair is its
best seller, the company continues to explore options to
enhance the profitability on the chair. The primary
objective of the “Complaint Seat-Recline Mechanism
for a high-end Office Chair” project is to minimize the
cost of the chair to improve its profitability and market
position. Currently the chair retails for $1086 to $3192
depending on the customer specified features.
Lowering the manufacturing costs and other associated
costs would significantly increase the profits generated
by the chair and make the chair more competitive in the overall market.
Figure 1: Current Knoll Life Chair
Knolls Furniture Company communicated to us that they would like us to provide them with a
new design of their office chair based on the current chair’s functions. They would like the chair
to cost less to manufacture and contain fewer parts. At the same time they would also want the
new chair design to maintain all the major functions the current chair design possesses. These
functions primarily include the back upright tilt lock, movable seat pan, and synchronized
recline. The major customer requirements of our series 400 office chair redesign as dictated by
our sponsor are listed below:
1. Low cost
2. Fewer parts.
3. Reliable
4. Maintain synchronized recline feature
5. Maintain back upright tilt lock
6. Maintain sliding seat pan
7. Easy to integrate into the current chair design
The main direction for the redesign of the office chair is to convert the current synchronized
recline and upright tilt lock mechanisms composed of conventional multipart mechanisms into a
single piece compliant mechanisms. The complaint mechanisms will result in significant
reduction of the number of parts needed for the assembly of the chair which would in turn
simplify the assembly process. The project is aimed at creating a successful integration of the
new synchronized recline compliant mechanism, upright tilt-lock compliant mechanism, and
other redesigned subsystems onto the existing chair to drastically reduce the production cost of
the chair.
3
CORRESPONDING ENGINEERING SPECIFICATIONS
General Engineering Specifications for Concept Generation
Based on the analysis of the original customer requirements from Knoll Furniture Company and
our study of the current chair’s mechanisms, we derived several major engineering specifications
for our new office chair design.
Simpler Synchronized Recline Mechanism: In order to significantly reduce the cost of the
chair from a manufacturing perspective, the chair’s synchronized recline mechanism for raising
and lowering the seat pan in response to the inclination angle of the back seat has to be
simplified. The reduction in the number of parts from this mechanism through compliant design
will significant cut the fabrication and assembly cost for the chair.
Simpler Upright Tilt-Lock Mechanism: The customer has expressed concerns with the current
upright tilt-lock system for the chair’s back. It is currently a complicated system containing
many moving parts. Replacing this mechanism using compliant system would reduce the number
of parts and lead to a significant cost reduction while improving the reliability of the locking
system as required by the customer.
Simple Assembly: The new chair design should be simple and quick to assemble even for the
most mediocre operator. Redesign of the chair for simple assembly would lead to a reduction in
the production cost of the chair through reduced labor and savings from the reduction of part
inventory. Simple assembly would also make repairing defective chairs faster and cheaper.
Maintenance of Functionality: While adequately meeting the improvement requirements that
the customer want, the design must also maintain the major functions that the current chair
possess. Some of the major functions are the synchronized recline, upright tilt-lock, and movable
seat pan. Thus the new chair design must be able to maintain the ±16 degrees back rotation,
perform the same synchronized recline motion as the current chair does, and have the ability of
being locked in the upright position.
Capable of Entering Full-scale Production: The current product is one of our customer’s best
selling products in the high-end office chair market. There is a significant demand that our
customer must meet and as result, the redesigned product must be capable of being produced at
the same scale of production as the current product. The customer required us to provide a
redesign of the chair that can be easily implemented onto the current chair. Thus our redesigned
office chair must be able to enter full-scale production at a relatively low product introduction
cost.
High Reliability and Safety: The new chair design must be able to support the 200 lb weight
with little possibility of failure, like the current chair.
4
Specific Design Characteristics
Through analyzing the general engineering specifications that were translated from the original
customer demand, we derived a set of specific part characteristics that can fully satisfy the needs
of our customer.
Quality Function Deployment for Specific Design Characteristics: Based on our new QFD
analysis, shown in Figure 2 on p. 6, we determined the key design characteristics that will meet
the engineering specifications to satisfy the original requirements of our customer. They are
ranked in order of importance below:
1.
2.
3.
4.
Feasible to manufacture at a low cost
Compliant synchronized recline mechanism
Compliant upright tilt-lock mechanism
High safety factor
Feasible for Manufacturing at a Low Cost: The key issue surrounding the parts in our new
design is cost. The customer’s primary requirement is the reduction of the chair’s production
cost. As a result, it is most critical that every part of our design has a low overall manufacturing
cost and product introduction cost (manufacturing cost encompasses value-adding operation,
logistics, overhead, setup, material, and labor cost.)
Compliant Synchronized Recline System: Having a compliant synchronized recline system is
the second most important characteristic for our design. This feature would reduce the 4-bar
linkage system composed of 4 parts in the original design down to 1 part composed of a fully
compliant 4-bar mechanism. This would drastically simplify the synchronized recline
mechanism, reduce assembly time, reduce the number of main parts in the chair, provide the
same reliability as the current design, and provide for exactly the same range of motion as the
original design. This new compliant mechanism can also be feasibly produced on a large scale
with relatively low production introduction cost.
Compliant Upright Tilt-Lock system: Having a compliant upright tilt-lock system is the third
most important characteristic for our design. This feature would significantly reduce the
complexity of the locking mechanism currently in place. The redesign will lead to a shorter
assembly time, fewer parts, and easy initiation of production. The reliability of the system will
not be affected.
High Safety Factor: To maintain reliability, our design must provide for the advised weight
limit on the current chair. As result, each system or component must have a high safety factor.
5
Figure 2: QFD for Office Chair Redesign
FUNCTIONAL DECOMPOSITION
Synchronized Recline Mechanism
The synchronized recline mechanism is activated when a force is applied onto the back-rest of
the chair causing the mechanical energy to transform and transfer. The transferred mechanical
energy moves throughout the four-bar linkage as torque and rotational velocity. The energy
displaces the seat pan into a raised parallel position with the ground. The transformed
mechanical energy turns into potential energy that is stored in the torsion bars. Once the force on
the back-rest is released then the stored potential energy will transform back into mechanical
energy and return the four-bar linkage into its original position. Functional decomposition of
synchronized recline mechanism is shown in Figure 3 on page 7.
6
Figure 3: Functional Decomposition of Synchronized Recline Mechanism
Upright Tilt Lock Mechanism
To control the upright tilt lock mechanism a force is applied from the cable actuator which in
turn causes a displacement at the end of the cable. The end of the cable is a block with teeth that
is in sync with gears. The displacement causes the gears to rotate and the rotation causes links
that are attached onto the gears to displace. The links are attached to stoppers that move in and
out of slots. Depending on the position of the stoppers the chair will either be locked or free to
recline. Functional decomposition of the locking system is shown in Figure 4 below.
Figure 4: Functional Decomposition of Locking System
CONCEPT GENERATION & CONCEPT SELECTION
Concepts for Synchronized Recline Mechanism
Based on the customer requirements and our general engineering specifications, we generated a
total of 7 concepts. Using the Pugh chart analysis shown in Table 1 on page 8, with the current
design as the baseline, we selected the top 3 concepts to explore further for our design. The
concepts selected were numbers 3, 5, and 7, each with its individual advantages and drawbacks.
7
Table 1: Pugh Chart Analysis for Preliminary Concept Selection
Concepts
selection criteria
1
2
3
4
5
+
+
+
+
+
Low Cost
0
0
0
0
Maintain Synchronized Recline
0
0
0
0
0
Maintain Sliding Seat Depth
+
+
+
+
+
Fewer Parts
0
0
0
Maintain Back Upright Tilt Lock
0
0
0
Reliability
+
+
+
+
Easy to integrate
3
3
3
2
3
Sum +'s
2
2
4
3
4
Sum 0's
2
2
0
2
0
Sum -'s
1
1
3
0
3
Net SCORE
4
4
1
4
1
Rank
No
No
Yes
No
Yes
Continue?
6
0
0
0
+
0
0
1
5
1
0
4
No
7
+
0
0
+
0
0
+
3
4
0
3
1
Yes
Concept 3: Concept 3 is a partially-compliant linkage system that only has 3 links connected by
thin flexural pivots. The input link is activated by the reclining motion of the chair’s back.
Acting as a lever, the downward force on the input link forces the whole mechanism to rotate
with the fulcrum at point A in Figure 5 below. This in turn achieves the parallel upward
movement desired for the seat pan. The links of this compliant linkage mechanism are attached
to the seat pan and pinned to the seat base (ground) at points A and D in Figure 5.
Figure 5: Design Concept 3
Figure 6: Design Concept 7
Concept 5: In this concept, the back-rest is connected to the base of the seat by a pivot joint in
order to go back and forth. Once force is applied to the back-rest to push it backwards, the
compliant mechanism connected to the back-rest will change from a parallelogram shape to a
rectangle shape. This will in turn cause the seat pan fixed on the mechanism to rise and be
horizontal to the ground, as shown in Figure 7 on page 9. The range of motion that this concept
can create is questionable and the extra size and cost associated with the extra supporting links
may pose a problem for this design concept.
8
Figure 7: Design Concept 5
Concept 7: This concept uses a compliant mechanism to replace the four-bar linkage while
maintaining the synchronized recline function of the current chair. Part A, shown in Figure 6 on
page 8, is attached to the seat base using four bolts. When the ground attached back is reclined, it
pushes down the two bars of part A. Then, the rest of linkages follow their own paths and
eventually lift up the front part of the seat. This design has the advantage of full compliancy and
full functionality (in terms of the range of motion it is capable of). However, this design concept
could be further simplified if the drive link between the chair’s back-rest and the mechanism is
improved.
Concepts for Upright Tilt-Lock Mechanism
The current upright tilt lock
system consists of six
plastic parts that can easily
fall apart. The new concept,
shown in Figure 8, will
replace the current
mechanism with a
compliant one-piece
mechanism. The new
compliant system will work
by compressing the middle of the mechanism with a cable actuator. The actuator will be
connected through the areas where the forces are shown in Figure 8. When the actuator
compresses the middle, this will displace the sides of the mechanism outward and into locking
slots, thus keeping the chair in one position. The only problem with the new compliant
mechanism is the repositioning of the cable actuator. Currently the cable actuator is positioned
parallel to the displacement of the mechanism, but in order to compress the middle of the
mechanism the cable will have to be positioned perpendicularly. The compliant mechanism will
reduce the number of parts from nine to three, since the blockers on both ends will be kept due to
its complex shape. The compliant mechanism will be beneficial in the assembly of the chair
because it will reduce the number of parts which reduces overhead and assembly time.
Figure 8: Locking System Design Concept
9
Design for Easy Assembly
Knoll, our sponsor, has asked us to
simplify the assembly process for
the torsion spring bars. Therefore,
the concept shown in Figure 9
reduces not only the number of
parts but also the assembly time.
There are two holes on one side of
the part, so that the two spring bars
can be slid into place. Then the
spring bars will be locked in by
retaining rings. There should not be
any problems from the adjustable torsion bar by implementing this manufacturing process. The
adjuster, shown in Figure 10 on page 11, will also have a hole drilled through the area for the bar
to be inserted.
Figure 9: Back Cast Concept
FINAL CONCEPT SELECTION
When choosing the best concept design, each concept was scored and ranked from the criteria
based on customer requirements as shown in Table 1 on page 8. Concepts 3, 5, and 7 have
advantages of low cost due to fewer parts and being easy to integrate onto the existing chair.
These designs also do not have significant negative effects to the current chair. All three designs
have the same net score and similar concept which is replacing four-bar linkage by compliant
mechanism. Therefore, we optimized our final design by combining the above concepts (3, 5,
and 7) together as shown in Figure 10 on page 11. Since compliant mechanism can store the
energy required to move back to an upright position, the torsion bars can be removed from the
back cast. After calculating the energy stored in the torsion bars we can design our α-design to
duplicate this feature. The α-design reduces 17 parts, which decreases the manufacturing cost
and simplifies assembly process, and the design can be easily integrated onto the current chair.
The drawback in our final design is that the thicknesses of the segments are relatively small, so
that those segments have a relatively high likelihood of failure. This makes the compliant
mechanism relatively less reliable than the conventional steel casted 4-bar linkage system.
Final Concept Description
The “Alpha Design” shown in Figure 10 on page 11 is a combined redesign of concepts 3 and 7. It is
called a pseudo-rigid-body model. Theoretically, the pseudo-rigid-body model concept is used to
model the deflection of flexible members using rigid-body components that have equivalent forcedeflection characteristics. Thus, the design uses the rigid links from the original four-bar linkage
system of the current chair while the joints in the current chair are replaced by the Small Linkage
Flexural Pivot (SLFP) at the same position. This will allow a full range of motion for the SLFP
segments so it will not be inhibited by the rigid links. The complaint mechanism will only work if the
SLFP segments are in tension and should not experience bending moment at the initial position.
Therefore, the segment orientations have to be the same with that of force on the current four-bar
linkages.
10
Another advantage of using SLFP segments is it can discard the torsion bars by storing the same
amount of energy needed to retain the original position of the current chair. By doing so, the
torsion bars can be replaced and there would be no need to implement design concept 9.
Figure 10: Final Concept for Compliant Synchronized Reclined System
Figure 11 shows the final design for the compliant upright tilt lock system. This final design will
work by compressing the middle of the mechanism with the cable actuator. The cable goes
through the holes of the mechanism. The initial position of the mechanism is unlocked position,
when the cable actuator is pulled the compliant mechanism will be compressed so that the link
connected to the mechanism will be extended. This will displace the blocks on each side and
lock the chair from reclining.
Figure 11: Final Concept for Compliant Upright Tilt-Lock System
ENGINEERING DESIGN PARAMETER ANALYSIS
Compliant Synchronized Recline Mechanism Parameter Analysis
Determination of Dimensions and Materials of SLFP Segments and Rigid Links: Using
ADAMS software, we built the original model and compliant model as shown in Figure C1 and
C2 in Appendix C. To determine the dimensions of the SLFP segments, based on the theory of
solid mechanics and compliant mechanisms, we computed the bending and tension stress using
11
Eθh
2l
F
=
hd
σ bending =
Eqn(1)
σ tension
Eqn(2)
where E is young’s modulus of the selected material, θ is angular motion range, h and l is the
thickness and length of SLFP segment, F is the applied force, d is the width of SLFP segment.
To compute the energy required to be stored on SLFP segments, we calculated the energy stored
on the torsion bars in the current chair with an assumption that the behavior of torsion bars is the
same as a linear spring. The following equations were used to calculate the spring constant k.
Fl 3
y max =
48 EI
F = kymax
Eqn(3)
Eqn(4)
where ymax is the maximum deflection of each end of the torsion bars, F is the applied force
transferred to the center of the torsion bars, E is the young’s modulus of steel, I is the second
moment of area, l is the length of the torsion bars. From equation (3) and (4), k was determined
to be following:
48 EI
Eqn(5)
k= 3
l
Then, the energy stored on the torsion bars, Ebars, was determined by:
Ebars =
1
24 EIymax
2
kymax =
2
l3
2
Eqn(6)
Based on the theorem of compliant mechanism, the energy stored by the SLFP segments, ESLFP,
is determined by:
4
1 EI i 2
θi
E SLFP = ∑
Eqn(7)
i =4 2 li
We constraint that the energy stored on the torsion bars equals to that on the SLFP segments and
the safety factor is to be two. Then, based on material properties, such as yield strength and
Young’s modulus, the width, thickness, and length of the SLFP segments were determined from
using Equations 1,2, and 7 above. We ran the same calculations using different materials, shown
in Table 3 on page 13, to find a feasible material for the compliant design. The only material that
can be used for our design is E-glass. Microsoft Excel Solver TM was used to calculate all the
dimensions of SLFP segments with the given constraints. The constraints are as follows
1. The total energy stored on the SLFP segments should be equal to that on the torsion bars.
2. The length of the SLFP segments should be no longer than 20% of the adjacent rigid
bars.
12
3. The thickness of each SLFP segment should be no more than 10% of the thickness of
rigid links
The determined dimensions of SLFP segments using E-glass are shown in Table 2 below.
Table 2: Determined SLFP Segments Dimensions
SLFP dimensions
Segment 1(pivot 1)
Segment 2(pivot 2)
Segment 3(pivot 3)
Segment 4(pivot 4)
Length
(m)
0.020
0.015
0.020
0.024
Thickness
(m)
0.002
0.002
0.003
0.003
Width
(m)
0.025
0.025
0.025
0.025
Length Ratio
(Rigid bar to SLFP)
3.80
6.67
6.55
5.46
Safety Factor
2.09
2.08
3.90
2.29
The thickness and width of the rigid links were determined based on the current chair dimensions,
but the length of the rigid links changed because of the length and orientation of the SLFP
segments. Although the rigid link lengths were changed, the motion of the compliant mechanism
is the same as that of four-bar linkage on the current chair because the pivots of the four-bar
linkage remained at the same locations, which is the midpoint of SLFP segment. The material for
the rigid links was chosen to be Aluminum 7075 since manufacturing rigid links using E-glass is
more expensive than the manufacturing cost for the current four-bar linkages. Aluminum 7075
has relatively low density and high machining performance, and it is relatively cheaper than
other Aluminum alloys. The disadvantage of manufacturing our design using two different
pieces is that we cannot reduce 17 parts, as previously expected. We still managed to reduce nine
parts by replacing the four-bar linkage with our design.
Based on the application and the function of the compliant system, materials for SLFP segments
were chosen to maximize both flexibility and stiffness. According to the theory of compliant
mechanism, one criterion for us to choose the material for SLFP segments was to maximize the
strength-to-modulus ratio. The material with the highest strength-to-modulus ratio allows a larger
deflection before failure. Since a large load (human body weight) needs to be supported while
the compliant mechanism is functioning, the material for the design needs to be strong enough to
support the large force. Materials with high yield strength were considered and are listed in Table
3 with their material properties and the strength-to-modulus ratio.
Table 3: Ratio of yield strength to Young’s Modulus of materials for SLFP
Material
E (GPa) Sy(MPa) (Sy/E)×1000
steel (4140Q&T@400
207
1641
7.9
aluminum (7075 heat treated) 71.4
503
7
Titanium (Ti-13 heat treated)
114
1170
10
Beryllium copper (CA170)
128
1170
9.2
Kevlar (82vol%) in epoxy
86
1517
18
E-glass (73.3vol%) in epoxy
56
1640
29
Determination of Orientations of SLFP Segments: To apply SLFP segments, it is crucial that
every SLFP segments remain in tension during the entire motion. This implies that the segments
will have to be oriented in the same direction as the force acting on each of them. Therefore, we
13
determined the joint segment orientation through the use of force analysis of the compliant
mechanism.
The preliminary orientations of the four SLFP segments are determined by building the pseudorigid-body model from ADAMS. The SLFP segments are simulated by the combination of pin
joints and torsion springs. The stiffness constants of torsion springs are determined by the
maximum rotation angle of each segment, the dimensions, and the material properties
determined above. With assumptions that a total gravity force with the magnitude of 444 N
acting on the middle of the seat pan (rigid bar in the model) and the static motion to run the
model , the forces acting on each joints are conducted from the software as shown in Figure D1
to D8 in Appendix D.
Based on the static analysis from ADAMS, we were able to determine angles of the orientation
of each SLFP segment shown in Table 4.
Table 4: Force Analysis and Optimized Orientation of SLFP
SLFP1
SLFP2
SLFP3
SLFP4
Initial Force
Fx (N)
Fy (N)
Angle (◦)
-274.80 -164.00
30.83
-340.40
153.70
24.30
345.90
-287.70
140.25
-345.90
288.50
140.25
Fx (N)
-445.60
595.00
600.60
-600.60
Final Force
Fy (N)
Angle (◦)
-336.50
37.06
283.40
25.47
-158.10
165.25
158.90
165.25
Optimized
orientation
33.95
24.89
152.75
152.75
Compliant Upright Tilt Lock System Parameter Analysis
Determination of dimensions and materials: Before we determined the dimensions and shape
of the locking compliant mechanism, the dimensional constraints were decided based on the
available space of the current chair. The width and the height should be less than 50 mm and
75mm, respectively. At the final position of the mechanism, the movement of each side tip has to
be 12 mm to properly function as a lock system. The possible shape of the locking compliant
mechanism was a circular or a diamond shape. Since the compliant mechanism should have
flexure joints, a diamond shape was chosen as our locking mechanism shape: there are four
flexure joints at each side for the diamond shape, which will make the lock mechanism have two
positions (unlock and lock). Compliant mechanism stores energy inside the mechanism, but we
have to reduce the stored energy as small as possible because the human force required to
operate the current locking mechanism is less than 10N. Thus, the thickness of the body should
be the smallest without any failures when functioning as a lock system. The thickness of flexure
joints should be less than that of body to generate bending motion on the flexure joints. Based on
the constraints, we designed the locking compliant mechanism using Pro-Engineering and
simulated it using AnSys to test whether there are any failures and if the deformation of the body
is acceptable.
Based on Table 5 on page 15, we determined that the upright tilt lock system should be made
with Polyethylene. The reason for this is three fold. First, it has a very high ratio of strength to
modulus. Second, it is readily available, inexpensive, easy to process, and has a low density.
Third, it is also very ductile. This makes it well suited for living hinges because the material
must undergo large strains for millions of cycles.
14
Table 5: Ratio of yield strength to Young’s modulus of materials for Lock System
Material
Polyethylene(HDPE)
Nylon (Type66)
Polyethylene
E (Gpa) Sy(Mpa) (Sy/E) ×1000
1.4
28
20
2.8
55
20
1.4
34
25
Figure 12: CES Selector Result for Material Using for Compliant Lock System
15
Finite Element Analysis Using ANSYS
Stress Analysis for Compliant Synchronized Recline Mechanism: Figures 13 and 14 show
the deformation analysis and the stress analysis, respectively.
Figure 13: Deformation analysis for the final design of the Compliant Synchronized Recline
Mechanism shows that the final design meets the displacement requirement in the seat pan area
(an upward displacement of approximately 15 mm).
Figure 14: Stress Simulation shows maximum stress experienced on the Compliant
Synchronized Mechanism satisfies the Safety Factor requirement of the design.
16
Stress Analysis for Compliant Upright Tilt Lock system: Figures15 and 16 show the
deformation analysis and the stress analysis, respectively.
Figure 15: Deformation analysis for the final design of the Compliant Upright-tilt Lock
Mechanism shows that the final design meets the displacement requirement for Each Side to
Lock the Chair (a horizontal displacement of approximately 7 mm)
Figure 16: Stress Simulation shows maximum stress experienced on the Compliant Upright-tilt
Lock Mechanism satisfies the Safety Factor requirement of the design
17
Buckling Analysis for SLFP Segments of the Compliant Synchronized Recline Mechanism
We had a buckling test done on our synchronized compliant mechanism to make sure that the
SLFP segments remained in tension. If there is any compression in the SLFP segments, they will
buckle so we performed this test to validate that our design is correct and all the segments were
in tension. The resulting graph can be seen in Figure 17. There is no buckling because strain
energy increases up to the maximum load.
Figure 17: Buckling Test for Synchronized Recline Mechanism
FINAL DESIGN DESCRIPTION
Prototype Description
Compliant Synchronized Recline Mechanism: There will be two prototypes built for the
complaint synchronized recline mechanism. The first one will be a one-piece aluminum
prototype and the second one will have e-glass SLFPs with aluminum rigid bars. These two will
be the same size as the current four-bar recline mechanism. The positions of the joints of the
synchronized recline mechanism will also be the same as the current four-bar recline mechanism.
The main difference between the prototype and the final design is the prototype only has planar
forces and motions while the final design should be designed for integration onto the existing
chair. While the input force for the final design and our prototypes will have different
orientations in space, the output displacements should be the same. This is because the motion
of the current four-bar mechanism is planar and since the prototype is planar the motion can be
replicated.
Another difference between the prototype and the final design is in manufacturing. The
aluminum with E-Glass SLFPs prototype will have the E-glass bolted onto the rigid aluminum
links. In the final design, the E-Glass inserts should be press fitted in the aluminum links thus
having one solid piece instead of multiple pieces. Although the manufacturing of the prototype
will be with multiple pieces, if the final design is manufactured then it should be a one-piece
18
compliant mechanism. This will reduce the number of parts needed for the office chair and thus
reduce the costs associated with the chair.
The entirely aluminum prototype will be used to demonstrate a one-piece compliant mechanism.
Although it may not be fully-functional as the other prototype, its purpose is to display that the
motion of the system is still maintained with only one piece. The drawbacks with this prototype
are that the output displacement will not be as large as the other prototype and the SLFP
segments have a smaller safety factor.
Upright Tilt-Lock Mechanism: The prototype of the upright tilt-lock mechanism will be
manufactured by Robert Coury in the ME 450 Manufacturing Lab. The prototype will be a fullscale model and should be able to fit into the size constraints of the chair. The prototype will be
made out of delrin and should displace the same amount as the final design.
There are two differences between the final design and the prototype of the upright tilt-lock
mechanism. The first difference is that the prototype will not be shaped at the ends. The final
design should have some sort of ball form at its ends. The difference is due to the difficulty of
manufacturing rounding in 3D.
The other difference between the prototype and the final design is the placement of the cable
actuator. In the prototype the cable comes from the bottom of the prototype and pulls on the top
to compress it. In the final design, the cable actuator should push down on the top of the
mechanism to cause an outward displacement by the ends.
Since the current locking mechanism is planar the prototype should be able to replicate the
motions because it is also planar. The way the force is applied from the cable is arbitrary
compared with the final results. As long as the prototype displaces its ends a certain amount then
the way the force is applied should not matter. The prototype shows that given a specific force it
will displace a certain amount regardless of how the force was applied. The unshaped ends on
the prototype can also be disregarded because its only use is for displaying how the prototype
works in comparison with the current locking mechanism.
FINAL DESIGN
Compliant Synchronized Recline Mechanism
Prototype Design: Based on our analysis and design synthesis, we developed a detailed
prototype design for a compliant synchronized recline mechanism with E-glass small length
flexural pivots. The major components of this design are shown in Table 6 on page 20. The
complete bill of material for this design is shown in Table E1 of the Appendix E. As shown, our
prototype design is composed of 2 different materials. The rigid main bodies’ (1-4) material is
Aluminum 2024 whereas the small length flexural pivots’ (1-4) material is E-glass. The detailed
assembly drawing and engineering drawing for the prototype are shown on the following pages.
In addition to the prototype described above, we manufactured a one piece prototype made
completely of Al 2024 by water jet cutting using the same design as the E-glass prototype. This
19
prototype was meant to demonstrate the idea of design for no-assembly for our application with
limited reliability, safety, and functionality.
Table 6: Part list for Compliant Synchronized Recline Mechanism Prototype with E-glass SLFP
Part ID Quantity
Part Description
P01
1
Al 2024 Rigid Main Body Component 1
P02
1
Al 2024 Rigid Main Body Component 2
P03
1
Al 2024 Rigid Main Body Component 3
P04
1
Al 2024 Rigid Main Body Component 4
P05
1
E-Glass SLFP Insert 1 - 2mm(thickness) x 25.4mm(width) x 41.13mm(length)
P06
1
E-Glass SLFP Insert 2 - 2mm(thickness) x 25.4mm(width) x 34.93mm(length)
P07
1
E-Glass SLFP Insert 3 - 3mm(thickness) x 25.4mm(width) x 40.00mm(length)
P08
1
E-Glass SLFP Insert 4 - 3mm(thickness) x 25.4mm(width) x 43.74mm(length)
Figure 18: 3-D Drawing of the Prototype Compliant Synchronized Recline Mechanism
Figure 19: Exploded Assembly Drawing of Prototype Compliant Synchronized Recline
20
Figure 20: Synchronized Recline Mechanism Engineering Prototype Drawing
Figure 21: Al 2024 Rigid Main Body Prototype Manufacturing Component 1
21
Figure 22: Al 2024 Rigid Main Body Full Scale Manufacturing Component 2
Figure 23: Al 2024 Rigid Main Body Full Scale Manufacturing Component 3
22
Figure 24: Al 2024 Rigid Main Body Full Scale Manufacturing Component 4
Figure 25: E-Glass SLFP Segment Insert 1
23
Figure 26: E-Glass SLFP Segment Insert 2
Figure 27: E-Glass SLFP Segment Insert 3
24
Figure 28: E-Glass SLFP Segment Insert 4
Full Scale Production Design: Based on our prototype design, we developed a variant of the
complaint synchronized recline mechanism with E-glass small length flexural pivots that
eliminates the need for the screws and nuts. This design variant added tight slots onto parts P01,
P02, P03, and P04 that allow the E-glass SLFP inserts to be press fitted in during the assembly.
The part list of the design is shown in Table 7 below and the detailed engineering drawings are
shown in the following pages.
Table 7: Part list for Full Scale Production Design Variant of Compliant Synchronized Recline
Mechanism with E-glass SLFP
Part ID Quantity
Part Description
F01
1
Al 2024 Rigid Main Body Component 1
F02
1
Al 2024 Rigid Main Body Component 2
F03
1
Al 2024 Rigid Main Body Component 3
F04
1
Al 2024 Rigid Main Body Component 4
F05
1
E-Glass SLFP Insert 1 - 2mm(thickness) x 25.4mm(width) x 31.13mm(length)
F06
1
E-Glass SLFP Insert 2 - 2mm(thickness) x 25.4mm(width) x 24.93mm(length)
F07
1
E-Glass SLFP Insert 3 - 3mm(thickness) x 25.4mm(width) x 30.00mm(length)
F08
1
E-Glass SLFP Insert 4 - 3mm(thickness) x 25.4mm(width) x 33.74mm(length)
25
Figure 29: 3-D Drawing of the Full Scale Compliant Synchronized Recline Mechanism
Figure 30: Exploded Assembly Drawing of Full Scale Compliant Synchronized Recline
26
Figure 31: Al 2024 Rigid Main Body Full Scale Manufacturing Component 1
Figure 32: Al 2024 Rigid Main Body Full Scale Manufacturing Component 2
27
Figure 33: Al 2024 Rigid Main Body Full Scale Manufacturing Component 3
Figure 34: Al 2024 Rigid Main Body Full Scale Manufacturing Component 4
28
Figure 35: E-Glass SLFP Segment Full Scale Manufacturing Insert 1
Figure 36: E-Glass SLFP Segment Full Scale Manufacturing Insert 2
29
Figure 37: E-Glass SLFP Segment Full Scale Manufacturing Insert 3
Figure 38: E-Glass SLFP Segment Full Scale Manufacturing Insert 4
30
Validation of Full-scale working Prototype: Our prototype will validate the general degree of
rotational motion that the final design can achieve through physical testing. It will also validate
the synchronized displacement level of the seat plan due to the rotation of the backrest. Finally, it
will validate that the concept of applying compliant mechanism design in the synchronized
recline mechanism of our current chair is feasible.
However since the prototype was not integrated onto the chair, we cannot be certain aside from
the results of our FEA model that our final design will not fail in real use under the predicted
maximum load. In addition, the reliability of the final design cannot be validated with the
prototype.
Final Design Operation: The rigid main body component P04 of our final design is fixed to the
base of the chair. When an input force is placed on the backrest of the chair, the backrest would
rotate. As a result of this rotation, rigid main body part P03 would rotate correspondingly. This
rotation is made possible by the small length flexural pivot component P05’s deformation. As a
result, the rigid main body part P02 pinned to the seat pan is displaced upward by the small
length flexural pivot component P06’s motion (induced by the motion of P03 that it is bonded
to). The displacement of part P02 in turn forces rigid main body component P01 to rotate
through both small length flexural pivot components P07 and P08. P07 is bonded on one end to
P02 and on the other end bonded to P01. P08 is bonded on one end to P04 (fixed rigid main body
component) and on the other end to P01. Hence as a result of the initial rotation of the backrest,
the seat pan fixed to component P01 is displaced upward allowing for the desired synchronized
motion.
Expected Level of Performance: We expect the prototype to be able to successfully achieve
±16◦ of rotational motion while achieving similar dynamic motion pattern as the current
mechanism under various different loads without failing. We will test the prototype design
through FEA simulation at different loads. In additional we will test the physical prototype to see
the maximum achievable angle of rotation without yielding any of the E-glass components.
These simulation and physical tests will ensure that our final design is accurate in meeting the
functionality needs of our customer.
Compliant Upright Tilt Lock Mechanism
Prototype Design: Based on our analysis and design synthesis, we developed a detailed design
for our compliant upright tilt lock mechanism. The design is composed of only 1 part whose
material is high density polyethylene.
Deviation of Physical Prototype from Full Scale Design: Two full-scale working prototypes
were made, one from HDPE and the other from Delrin. The HDPE working prototype was
manufactured using laser cutter. As a result, the material was melted and the nominal material
property of HDPE was altered. Thus, we made another working prototype from Delrin. Since
there is a material property difference between Delrin and HDPE, the working prototype requires
a larger input force to deform but offered improved energy storage.
Validation of Full-scale working prototype: We plan to validate the working prototype
through FEA simulation and physical testing. In our FEA simulation we plan to apply a
31
reasonable load that the current locking switch is capable of withstanding and determine the level
of displacement at its ends. In our physical test, we plan to apply a reasonable load by hand
through a metal cable attached to the working prototype to test for the level of displacement its
ends.
Final Design Operation: Our final design will achieve displacement at its two ends which are
attached to stopper blocks to lock the back-rest in place.
When the user attempts to lock the back-rest in the upright position, they will press down on the
locking lever at the side of the chair. This lever with a bimodal spring inside will pull the cable
connected to the upright tilt-lock mechanism in the forward direction compressing the compliant
upright tilt-lock mechanism. As a result of this compression, our mechanism would change shape
through its flexural joints and extend the two ends attached to the stopper blocks outward into
slots. The locked position is maintained by a bimodal spring inside the lock lever.
When the lock lever is pull upward and released, the compression force on the upright tilt-lock
mechanism will be released and the energy stored in the mechanism will return the mechanism to
its original position.
Expected Level of Performance: The expected level of performance from a functionality point
of view is quite high. Our final design should be able to achieve enough displacement at its ends
to allow for the stopper blocks to fit into the slots with the reasonable amount of input force.
However, there will be fatiguing issues as the material of the mechanism is a polymer. The
fatigue life of the design maybe relatively low.
Figure 39: 3-D Drawing of the Compliant Upright Tilt-Lock Mechanism
32
Figure 40: Engineering Drawing of the Compliant Upright Tilt-Lock Mechanism
MANUFACTURING PLAN
Compliant Synchronized Recline Mechanism
Prototype Manufacturing Plan: Based on our prototype design, we developed a plan upon
which our prototype was manufactured. The general process by which we manufactured our
prototype is shown in the flow chart in the figure below.
Figure 41: Compliant Synchronized Recline Mechanism Prototype Manufacturing Plan Block Diagram
33
The detailed step by step manufacturing for our major components are shown in Table 8 below.
Table 8: Manufacturing Plan for Synchronized Recline Prototype with E-Glass
Part #
P01
Part Name
Al 2024
Rigid Main Body 1
P02
Al 2024
Rigid Main Body 2
P03
Al 2024
Rigid Main Body 3
P04
Al 2024
Rigid Main Body 4
P05
E-glass SLFP 1
P06
E-glass SLFP 2
P07
E-glass SLFP 3
P08
E-glass SLFP 4
Sequence #
Tool
1
Band Saw
2
Vise
3
3/8" End Mill
4
#36 Drill Bit
5
6-32 Tap
6
Vise
7
3/8" End Mill
8
#36 Drill Bit
9
6-32 Tap
10
Sand Paper
1
Band Saw
2
Vise
3
3/8" End Mill
4
#36 Drill Bit
5
6-32 Tap
6
Vise
7
3/8" End Mill
8
#36 Drill Bit
9
6-32 Tap
10
Sand Paper
1
Band Saw
2
Vise
3
3/8" End Mill
4
#36 Drill Bit
5
6-32 Tap
6
Vise
7
1/4" End Mill
8
Vise
9
#36 Drill Bit
10
6-32 Tap
11
Sand Paper
1
Band Saw
2
Vise
3
3/8" End Mill
4
#36 Drill Bit
5
6-32 Tap
6
Vise
7
3/8" End Mill
8
#36 Drill Bit
9
6-32 Tap
10
Sand Paper
1
Band Saw
2
Mill
3
E Drill
1
Band Saw
2
Mill
3
E Drill
1
Band Saw
2
Mill
3
E Drill
1
Band Saw
2
Mill
3
E Drill
Procedure
Cut part out of the water jet cut base shape (300FPM)
Clamp part in vise on mill table with bonding surface 1 parallel to mill table
Mill bonding surface 1 to create machined surface (1400 RPM)
Drill 0.217mm holes in design locations on bonding surface 1 to 15 mm depth (600 RPM)
Tap holes
Clamp part in vise on mill table with bonding surface 2 parallel to mill table
Mill bonding surface 2 to create machined surface (1400 RPM)
Drill 0.217mm holes in design locations on bonding surface 2 to 15 mm depth (600 RPM)
Tap holes
Sand part surface and clean up the part
Cut part out of the water jet cut base shape (300FPM)
Clamp part in vise on mill table with bonding surface 1 parallel to mill table
Mill bonding surface 1 to create machined surface (1400 RPM)
Drill 0.217mm holes in design locations on bonding surface 1 to 15 mm depth (600 RPM)
Tap holes
Clamp part in vise on mill table with bonding surface 2 parallel to mill table
Mill bonding surface 2 to create machined surface (1400 RPM)
Drill 0.217mm holes in design locations on bonding surface 2 to 15 mm depth (600 RPM)
Tap holes
Sand part surface and clean up the part
Cut part out of the water jet cut base shape (300FPM)
Clamp part in vise on mill table with bonding surface 1 parallel to mill table
Mill bonding surface 1 to create machined surface (1400 RPM)
Drill 0.217mm thru holes in design locations on bonding surface 1 (600 RPM)
Tap holes
Clamp part in vise on mill table with surface 3 parallel to mill table
Mill Intrusion as shown in design
Clamp part in vise on mill table with surface 2 parallel to mill table
Drill 0.217mm thru holes in design locations on surface 2 (600 RPM)
Tap holes
Sand part surface and clean up the part
Cut part out of the water jet cut base shape (300FPM)
Clamp part in vise on mill table with bonding surface 1 parallel to mill table
Mill bonding surface 1 to create machined surface (1400 RPM)
Drill 0.217mm holes in design locations on bonding surface 1 to 15 mm depth (600 RPM)
Tap holes
Clamp part in vise on mill table with bonding surface 2 parallel to mill table
Mill bonding surface 2 to create machined surface (1400 RPM)
Drill 0.217mm holes in design locations on bonding surface 2 to 15 mm depth (600 RPM)
Tap holes
Sand part surface and clean up the part
Cut to general length and width as marked on E-glass sheet
Create squared edges and mill to 25.4mm X 41.13mm
Drill 1/4" thru holes at design locations X 4
Cut to general length and width as marked on E-glass sheet
Create squared edges and mill to 25.4mm X 34.93mm
Drill 1/4" thru holes at design locations X 4
Cut to general length and width as marked on E-glass sheet
Create squared edges and mill to 25.4mm X 40mm
Drill 1/4" thru holes at design locations X 4
Cut to general length and width as marked on E-glass sheet
Create squared edges and mill to 25.4mm X 43.74mm
Drill 1/4" thru holes at design locations X 4
We then assembled the final prototype together using the following procedure.
Table 9: Assembly of Synchronized Recline Prototype with E-Glass
Sequence #
1
2
3
4
5
6
7
8
Tool
Philip Screw Driver
Philip Screw Driver
Philip Screw Driver
Philip Screw Driver
Philip Screw Driver
Philip Screw Driver
Plier
Philip Screw Driver
Procedure
Place 2 washers between 2 6-32 x 3/4" round head screws and P07 and screw P07 onto P02
Place 2 washers between 2 6-32 x 3/4" round head screws and P07 and screw P07 onto P01
Place 2 washers between 2 6-32 x 3/4" round head screws and P06 and screw P06 onto P02
Place 2 washers between 2 6-32 x 3/4" round head screws and P06 and screw P06 onto P03
Place 2 washers between 2 6-32 x 1" round head screws and P03 and screw P05 onto P03, add nuts
Place 2 washers between 2 6-32 x 3/4" round head screws and P08 and screw P08 onto P04
Place 2 washers between 2 6-32 x 3/8" hexagonal head screws and P08 and screw P08 to P01
Place 2 washers between 2 6-32 x 3/4" round head screws and P05 and screw P05 onto P04, close mechanism
34
Full Scale Production Manufacturing Plan: In our prototype design, we used screws extensive
to bond the E-glass SLFP inserts to the rigid main body components which is not feasible in
terms of cost for the full scale production. As a result, we developed design variant for full-scale
production that eliminated the needs of screws and nuts through the addition of slots for the Eglass inserts. For full-scale production, the 4 rigid aluminum components F01 to F04 would be
die-cast using a single die. Additional holes should then be drilled on the components for
integration with the chair. The 4 E-glass SLFP’s would be stamped out of Prepreg E-glass sheets
with a capable stamping press that can make clean cuts through fiber glass. To assemble the Eglass and rigid main components together, the rigid components must first be frozen to widen the
tight slot. Afterward, the E-glass should be press fitted into the slots at their corresponding
locations and as the metal components warm up, they would expand and tighten the slots. This
would embed the E-glass inserts in the rigid main body components forming the final design.
The required assembly time for our mechanism is relatively short.
Compliant Upright Tilt Lock Mechanism
We developed 2 physical prototypes for our complaint upright tilt lock mechanism. The first one
we made was from HDPE plates. The manufacturing processes used to develop this prototype is
shown below.
Figure 42: Compliant Upright Tilt-Lock Mechanism Prototype Manufacturing Plan Block Diagram
Due to the poor surface finish on our first prototype, we manufactured another prototype using
CNC Mill with the help of Mr. Bob Coury. The material of this prototype was not the HDPE that
our design was based on as it was made from Delrin.
TEST RESULTS
We validated our designs by manufacturing prototypes that had the same concepts and ideas.
The prototypes were tested by applying enough force to allow for a full range of motion. After
testing our prototypes we can validate that our final designs will work. We also used Ansys for
finite element analysis to conduct simulations with varying forces on our designs. The only
disadvantage of using Ansys is that we used a linear model instead of a non-linear model so if
the input was doubled, the output would be doubled too.
35
Compliant Synchronized Recline Mechanism
Upon completing the manufacturing of both the one-piece aluminum prototype and the two-piece
aluminum/E-glass prototype we fastened it onto the display boards and conducted tests on them.
By applying a horizontal force on the back-rest we were able to cause an elastic deformation on
the prototypes. The one-piece aluminum prototype had a vertical displacement of 0.5 cm given a
range of five degrees on the back-rest. The other prototype with E-Glass SLFP segments had a
vertical displacement of 1.2 cm given ±12 degrees of motion.
Using AnSys for finite element analysis, the one-piece aluminum prototype had a vertical
displacement of 4.45 mm given a force of 67.5 N on the back-rest and 150 N on the seat pan. If
a greater force was applied, the design would fail due to plastic deformation. The E-Glass
prototype had a vertical displacement of 1.52 cm with a horizontal force of 180 N and a vertical
force of 400 N on the seat pan. Since E-Glass has different material properties (Young’s
Modulus, Yield Strength) we can apply greater loads on the E-Glass model without worry about
failure. The results from FEA and the prototypes can be seen in Table 10 below.
Table 10: Comparison of the Results from Real and Virtual Compliant Recline Prototype
Range of
Horizontal Vertical
Motion
Force (N) Force (N)
(degrees)
FEA Aluminum
67.5
150
Prototype Aluminum
5
FEA
E-Glass
67.5
150
FEA E-Glass
180
400
Prototype E-Glass
12
Type
SLFP
Material
Displacement
(cm)
Von Mises
(MPa)
0.445
0.5
0.57
1.52
1.25
273
265
770
-
Yield
Strength
(GPa)
0.28
0.28
1.6
1.6
1.6
The discrepancies of the results are due to many factors such as uneven thicknesses for the SLFP
segments on both prototypes, the force applied to the backrest is approximate, and losses due to
friction. The SLFP segments were filed down in the one-piece aluminum prototype because the
current dimensions could not be manufactured with water-jet cutting. This may have lead to
uneven SLFP segments because filing is very rough and inaccurate. For the E-Glass SLFP
segments the thicknesses were all the same because we only had one E-Glass Prepreg sheet. We
had used different thicknesses in the finite element analysis with our final design. Another factor
that contributed to the discrepancies was the measurement of forces. Since there is not an
accurate way to measure forces in real life, we cannot precisely determine the force that we
applied to the back-rest/lever. The different forces will cause some variations in the results.
Finally, the E-Glass SLFPs were screwed onto the rigid links in the prototype so some losses due
to friction can occur since the SLFPs were not precisely positioned.
Upright Tilt-Lock Mechanism
After the upright tilt-lock mechanism was completed, tests were done to determine how effective
the design is. Compressing the middle of the prototype to a significant extent, the resulting
displacement from the ends of the system was 0.7 cm in each direction. From finite element
36
analysis, AnSys simulated a displacement of 0.657 cm in each direction given a 10 N force. The
results can be seen in Table 11.
Table 11: Comparison of the Results from Real Virtual Compliant Lock System Prototype
Compressive Force Displacement
Von Mises
Yield Strength
Type
Material
(N)
(cm)
(MPa)
(Mpa)
FEA
Delrin
10
0.657
51.26
60
Prototype
Delrin
0.7
60
The discrepancies in results could be due to the approximate forces applied to the prototype.
Since a certain force can not be accurately applied on an object, any change in the force will
result in a different displacement for the prototype. Another factor that could have caused the
discrepancies is frictional losses. Since the prototype is lying on the display board, the friction
forces generated by the prototype rubbing against the wooden board can cause the displacements
to be smaller than it actually is.
DESIGN CRITIQUE
Synchronized Recline Compliant Mechanism
The strength of the final design is lowering the manufacturing cost by discarding nine parts from
the current four-bar linkage mechanism. An additional strength is solving the problem of the
complicated assembly process of the torsion bars. This was done by using the compliant
mechanism to store the energy required to bring the chair into its upright position. The strengths
of our design are summarized in the Table 12 below.
Table 12: Strengths of Final Design
Strengths of Design
Eliminating 9 parts -> Reduces manufacturing cost
Storing energy -> Eliminates torsion bars -> Reduces assembly time -> Reduces
manufacturing cost
However, our final design is not entirely one-piece, which is different from the original concept,
because manufacturing our entire design out of E-glass would have been more expensive than
the cost for the current chair. E-glass is the only feasible material for the Small Length Flexural
Pivot (SLFP) given the geometric constraints. The constraints were found by the dimensions of
current four-bar linkage on the chair (integrating the new design into the current chair is the
customer requirement). Without limiting the geometric dimensions on the new design, other
materials such as Titanium and Spring Steel could be used for SLFP segments, which cause the
new design to be made entirely out of one piece. Another weakness of our design is that the final
motions of the seat pan differ with different input forces, as shown in Figure 43 on page 38. The
output displacement is proportional to the input force because we used a linear model in AnSys.
This is realistic in some sense because we do not expect a child to be able to fully deflect the
back-rest. This problem can be solved by shortening the SLFP lengths with changing the
geometric constraint.
37
Table13: Weakness and Solutions of Final Design
Weakness
8 piece compliant
mechanism
Inconstant output motion
Solution
Changing dimensional constraint->Using spring steel
or titanium instead of E-glass
Changing dimensional constraint->Shortening SLFP
lengths
Figure 43: Linear relationship between the displacement of the seat pan and the input force for
the compliant synchronized recline mechanism
1.6
1.4
1.2
y = 0.0038x
Deflection (cm)
1
0.8
0.6
0.4
0.2
0
0
50
100
150
200
250
300
350
400
450
Vertical Force (N)
Upright Tilt Lock Compliant Mechanism
We were able to reduce the current locking mechanisms part count by three through using our
final compliant mechanism. This will result in reducing the manufacturing cost of the chair.
There is not any weakness for the new locking design except for the fatigue failure on SLFP.
Unfortunately, we were not able to determine any fatigue life for the compliant mechanism
because depending on its shape and forces the fatigue life would change even with the same
material.
INFORMATION SOURCES
Benchmarking
We benchmarked the Knoll Life Chair Series 400 against four other high-end office chairs:
Liberty Chair by HumanScale, Aeron Chair by Herman Miller, Think Chair by Steelcase, and an
Office Star Chair by Knoll. The Life Series 400 chair costs between 21-2142 % more than the
other chairs. The options available for the Life Series 400 chair are what cause the huge range of
38
prices. The Life Series 400 chair had the most features among all the chairs. Such features
included vertical adjustability, swiveling, adjustable arms, seat moves forward/backward, and
reclining.
In terms of weight the Series 400 chair (35 lbs) is in the middle of the competition because while
it is considerably lighter than the Office Star Chair (65 lbs) it is heavier than the Liberty Chair
(27 lbs). The Series 400 chair is among the best in terms of using recycled material. The chair
uses 64% recycled material but the Liberty chair uses an astonishing 90% recycled material.
Overall, while the Life Chair Series 400 has impressive functionality and specifications, the high
price tag of the chair is significantly higher than all other chairs and will surely deter some
customers.
CONCLUSIONS
Upon completion of the project, we can conclude that we have achieved the goals set forth. The
main motivation of the project was to reduce the cost of the current chair by limiting the number
of parts used while still maintaining the various functions of the chair. We reduced the
synchronized recline and the upright tilt-lock mechanisms’ part counts from 30 to 21 parts and 6
to 3 parts, respectively. This will help reduce the cost of manufacturing the chair as well as the
indirect costs such as overhead because it will reduce assembly time. The other focus of the
project was to maintain the current functions of the chair. We feel that this objective was
achieved due to the prototype test results that we obtained. The current chair has a positive
vertical displacement of 1.55 cm on the seat pan and the upright tilt-lock mechanism displaces
1.3 cm on each side. The prototypes that were tested had similar results. The compliant
synchronized recline prototype (with E-Glass SLFP segments) had a vertical displacement of 1.2
cm and the compliant upright tilt-lock prototype had horizontal displacements of 0.7 cm on each
side. Since we were unable to achieve the dimensions of the final designs in the prototypes we
feel that using finite element analysis we can simulate the real results. From FEA we were able
to obtain 1.51 cm of vertical displacement for the compliant synchronized recline mechanism
and 0.7 cm for the compliant upright tilt-lock mechanism. Based on these results we feel that we
have achieved the goals of the project because we reduced the number of parts which will in turn
reduce the cost of the current chair. We also maintained the current functions of the chair
because the results we received from our prototype were similar to the current chair’s attributes.
We recommend that distributed compliant mechanisms be explored in the future work and also
be able to integrate the designs onto the current chair.
RECOMMENDATIONS
We have two recommendations concerning the synchronized recline mechanism for Knoll and
future design teams. We feel that the development of a distributed compliant one-piece
mechanism should be explored. We also recommend the future teams to try and integrate the
design onto the chair. We did not have enough time, resources, or knowledge of manufacturing
to be able to integrate our design onto the existing chair.
The first recommendation is to explore the benefits of using a one-piece distributed complaint
mechanism. The distributed compliant mechanism will be able to store energy without the use of
39
SLFP segments. It will also have the luxury of being entirely one-piece. After manufacturing
our aluminum one-piece design we discovered the advantages of having a one-piece mechanism.
It is much simpler to manufacture a part out of one piece than having to deal with multiple
materials and how to integrate them with one another. This will save time and money to
whoever continues the project.
The only disadvantage in developing the one-piece design is the time spent on designing it.
Currently, there is not any software available that is able to design distributed compliant
mechanism. We had tried using a graduate student’s program but failed to achieve any desirable
results. We lacked the knowledge to be able to design a distributed complaint mechanism since
there are numerous variables to account for. The design will be very complicated and the future
teams needs to be weary of parasitic or unwanted motion. The main objective of the project was
to insure that the current functions of the chair were maintained, future teams must pay close
attention in matching the motion of the current chair.
The other recommendation is to integrate the design onto the existing chair. We did not have
enough resources (time and money) to be able to complete this task. First, we did not have
enough time because we also focused on redesigning the upright tilt-lock mechanism. Time
must be spent on properly measuring and designing the mechanism to be able to fit properly in
the current constraints. The other resource that we did not posses was enough funding to be able
to manufacture a 3d design. The costs of manufacturing our three prototypes already put us near
the max of our budget. It will be very costly to contract out a skilled machinist/manufacturer to
produce a product that has contours and variations in 3 dimensions. We feel that integration onto
the chair is possible and should be done in the future.
ACKNOWLEDGEMENTS
We would like to thank the people who helped us make our senior design project a success.
First, we want to thank Professor Sridhar Kota for sponsoring the project and guiding us along
whenever we got stuck. Next, we want to thank the graduate students, Michael Cherry,
Youngseok Oh, and Tanakorn Tantanawat, for helping us with the design process and the
validation of our project. Finally, we want to thank Robert Coury and Steve Emanuel for helping
us manufacture our prototypes.
REFERENCES
1. Goe, M. Moreno, A. Knoll Life Chair Series 400 Re-design Report. University of
Michigan, Ann Arbor. 2006.
2. Howell,L. Compliant Mechanisms John Wiley & Sons, Inc 2002
3. Li, S. Design Procedure of Compliant Gripper University of Michigan, Ann Arbor. 2005
40
APPENDIX A. Figure A1 Gantt Chart Diagram
41
Appendix B. Preliminary design concepts
Synchronized Recline Mechanism
Concept 1
Figure B1: Design Concept 1
Concept 1 presents a design that is a fully
compliant system with distributed
compliance that replaces the original
linkage mechanism of the synchronized
recline system. As the user leans on the
back of the chair, an input bar with a
expandable compliant joint G shown in
Figure 3 in the left pulls on the compliant
linkage mechanism and causes rotation of
its links which raises the seat pan upward
parallel to the seat base. The compliant
linkage mechanism is attached to the seat
pan and pinned to the seat base (ground) at
points C and F in Figure B1
Concept 2
Figure B2: Design Concept 2
Concept 2 presents a design that is a fully
compliant system with distributed compliance
that replaces the original linkage mechanism
of the synchronized recline system. The user’s
leaning back motion activates the input
linkage which is pinned to the back and
compliant jointed at point G in Figure B2 in
the left to the main compliant mechanism
structure. This forces the links of the main
compliant mechanism structure to rotate
which raises the seat up parallel to the seat’s
base (ground). The compliant linkage
mechanism is attached to the seat pan and
pinned to the seat base (ground) at points C
and F in Figure B2.
42
Concept 3
Figure B3: Design Concept 3
Concept 3 diverges from the previous 2
concepts (concept 1 and 2 on page 7) in that
it is a fully-compliant linkage system that
only has 4 links. The input link is activated
by the reclining motion of the chair’s back.
Acting as a lever, the downward force on
the input link forces the whole mechanism
to rotate with the fulcrum at point A in
Figure B3. This in turn achieves the parallel
upward movement desired for the seat pan.
The links of this compliant linkage
mechanism are attached to the seat pan and
pinned to the seat base (ground) at points A
and D in Figure B3.
Concept 4
This concept has the chair being naturally reclined because it will be the compliant
mechanism’s initial position. The compliant system will include the back rest with a seat
pan able to slide on top of the seating area, as shown in Figure B4. When the user sits up
properly in the chair, the compliant mechanism will compress into its second position and
act as a normal upright chair. A new locking mechanism will have to be designed to keep
the chair in the upright position, but as soon as the lock is released the chair will return to
its original reclined position. The compliant mechanism is designed to replicate the
traditional four bar linkage that was originally on the office chair. The manufacturing of
the concept will be a problem due to the unusual shape of the chair. Also, the integration
of the concept onto the existing chair will also cause a problem because the concept will
replace most of the current chair.
Figure B4: Design Concept 4
43
Concept 5
Figure B5: Design Concept 5
In this concept, the back is connected to the base of the seat by a pivot joint in order to go
back and forth. Once the back has been pushed backwards, the compliant mechanism
connected to the back will change from a diamond to a rectangle shape, so that the pad
fixed on the mechanism will go up and be horizontal to the ground, as shown in Figure
B5.
Concept 6
This concept is
composed of three
major parts: A rigid
frame which has a
pivot point at the
corner so that it can
be pushed back and
forth, a seat pad that
connected to the
frame with a pivot
joint at one side as
show in Figure B6,
and the compliant
mechanism bar which
connects the rigid
frame with the seat
pad. Once the frame
is pushed back, the
Figure B6: Design Concept 6
44
pad will also go up and be tilted. In order for the seat pad to be parallel to the ground, the
compliant mechanism assists the other side of the pad to go up.
Concept 7
This concept reduces four parts using a
compliant mechanism for the four-bar
linkage while maintaining all functions
of the current chair. The part A shown
in Figure B7 on the left is attached to
the seat base using four screws. When
the ground attached back is reclined,
the back pushes down two bars of the
part A. Then, the rest of linkages
follow their own paths and eventually
lift up the front part of the seat.
Figure B7: Design Concept 7
Upright Tilt Lock Mechanism
The current upright tilt locking
mechanism consists of six
plastic parts that can easily fall
apart. The new concept, shown
in Figure B8, will replace the
current mechanism with a
compliant one-piece
mechanism. The new
compliant system will work through compressing the middle of the mechanism. This will
in turn push the sides of the mechanisms outward and into slots, thus locking the chair in
one position. The only problem with the new compliant mechanism is the cable
assembly recline lock will need to be repositioned. Currently the cable assembly recline
lock is positioned parallel to the displacement of the mechanism, but in order to compress
the middle of the mechanism the cable assembly recline lock will have to be positioned
perpendicularly. The compliant mechanism will have a benefit in the assembly of the
chair because it will be just one complete piece instead of six separate pieces.
Figure B8: Locking System Design Concept
45
Design for Assembly
Knoll, our sponsor, has asked
us to simplify the assembly
process for the torsion spring
bars. Therefore, the concept
shown in Figure B9 reduces
not only the number of parts
but also the assembly time.
There are two holes on one
side of the part, so that the
two spring bars can be slid
into place. Then the spring bars will be locked in by retaining rings.
Figure B9: Back Cast Concept
46
Appendix C. Dimension analysis for SLFP segments of Compliant Synchronized
Recline Mechanism
Model used in the parameter analysis
Figure C1: Model for the original four-bar linkage recline mechanism
Recline Force
(90N)
Human weight
(400N)
Torsion Bar
Figure C2: Pseudo-rigid-body model for the Compliant recline mechanism
Recline Force
(90N)
Human weight
(400N)
Mechanism Parameters
Table C1: Angles of each pivot joint
Initial Angle
Final Position
Pivot Joint
(˚)
(˚)
57.60
73.60
pivot 1
143.27
131.24
pivot 2
28.75
34.46
pivot 3
130.38
118.71
pivot 4
47
Angle Range
(˚)
16.00
-12.03
5.71
-11.67
Angle range
(radians)
0.28
0.21
0.10
0.20
Using the force and energy equations mentioned in the report, energy and stress data was
calculated in the Excel spreadsheet as shown below.
Table C2. Energy and stress data calculated in Excel spreadsheet
Energy bending
SLFP
stored
stress
tension stress Total Stress Safety Factor
Segment
3.42E+06
7.85E+08
2.09E+00
1(pivot 1) 1.85E+00 7.82E+08
Segment
3.42E+06
7.87E+08
2.08E+00
2(pivot 2) 1.39E+00 7.84E+08
Segment
2.05E+06
4.21E+08
3.90E+00
3(pivot 3) 7.95E-01 4.19E+08
Segment
2.05E+06
7.15E+08
2.29E+00
4(pivot 4) 2.77E+00 7.13E+08
48
Appendix D. Force analysis for each pivot joint using ADAMS Model
Fig D1. Initial forces in X and Y directions on
Fig D2. Final forces in X and Y directions on
SLFP1
SLFP1
Fig D3. Initial forces in X and Y directions on
SLFP2
Fig D4. Final forces in X and Y directions on
SLFP2
Fig D5. Initial forces in X and Y directions on
SLFP3
Fig D6. Final forces in X and Y directions on
SLFP3
FigD7 . Initial forces in X and Y directions on
SLFP4
49
Fig D8. Final forces in X and Y directions on
SLFP4
Appendix E. Bill of Material
Table E1
Part ID
P09
P10
P11
P12
P14
P01
P02
P03
P04
P05
P06
P07
P08
Quantity Part Description
12
2
2
2
16
1
1
1
1
1
1
1
1
Part Source
6-32 x 3/4" round head UNC screws
University of Michigan
6-32 x 1" round head UNC screws
University of Michigan
6-32 x 3/8" hexagonal head UNC Screws
Plymouth Hardware
6-32 Machined Nut
University of Michigan
1/4" Inner Diameter Washers
University of Michigan
Al 2024 Rigid Main Body Component 1
Quasar Industries
Al 2024 Rigid Main Body Component 2
Quasar Industries
Al 2024 Rigid Main Body Component 3
Quasar Industries
Al 2024 Rigid Main Body Component 4
Quasar Industries
E-Glass SLFP Insert 1 - 2mm(thickness) x 25.4mm(width) x 41.13mm(length) University of Michigan
E-Glass SLFP Insert 2 - 2mm(thickness) x 25.4mm(width) x 34.93mm(length) University of Michigan
E-Glass SLFP Insert 3 - 3mm(thickness) x 25.4mm(width) x 40.00mm(length) University of Michigan
E-Glass SLFP Insert 4 - 3mm(thickness) x 25.4mm(width) x 43.74mm(length) University of Michigan
Part Cost (Each)
$0.00
$0.00
$0.17
$0.00
$0.00
$91.25
$91.25
$91.25
$91.25
$0.00
$0.00
$0.00
$0.00
Table E2
Part ID
L01
L02
Quantity Part Description
1
1
Part Source
DELRIN SHEET ½’’ THICK 12’’× 12''
POLYETHYLENE (HDPE) SHEET 3/8’’ THICK 12'' × 12''
50
University of Michigan
MCMASTER-CARR
Part Cost (Each)
$0.00
$0.00
51